Gable-top containers or containers are used widely for packaging milk, juices, and other foods, as well as a variety of other products. Such containers are made from sheet material which is heat-sealable to itself. The typical material is paper board coated on both sides with polyethylene or another heat-sealable material. The construction and parts of such containers and the blanks from which they are formed are described and illustrated, for example, in U.S. Pat. No. 4,744,467, issued to Tetra Pak International AB, and U.S. Pat. No. 4,775,096, issued to AB Tetra Pak. Those entire patents are incorporated by reference here.
Gable closures are conventionally sealed by folding the carton along preformed score lines to form an upstanding fin (or, for a bottom closure, a depending fin) having several thicknesses of heat sealable material. The fin is captured between an ultrasonic sealing horn and an anvil, and ultrasonic energy transducer to the joint through the horn causes the heat-sealable material in the fin to fuse, forming a seal.
One problem which such sealing apparatus must overcome is the difficulty of reliably applying a uniform sealing pressure, accurately located on the closure surfaces. This problem is particularly felt when the sealing apparatus has multiple, commonly driven ultrasonic horns and anvils for simultaneously sealing a corresponding number of cartons in one sealing stroke. Misalignment or other maladjustment of a single sealing horn or its corresponding anvil can result in an unacceptable carton reject rate or bring into question the integrity of the seals of products released for sale.
The anvil opposing a single ultrasonic transducer has previously been proposed to be mounted on a ball joint, so the anvil will self-adjust into parallelism with the face of the ultrasonic energy transducer when work is clamped between the anvil and transducer. This arrangement is shown in U.S. Pat. No. 3,661,661.
Another problem which carton sealing apparatus must address is the need to adjust the sealing pressure exerted by the apparatus, either when the machine is being reconfigured to seal a different type of package or to fine-tune the machine during a product run.
Yet another problem is how to provide a compact sealing station, particularly if both the anvils and the sealing horns are driven, and thus require separate drives. An independent drive on each side of the sealing station makes the machine wider than a machine in which only the horns or only the anvils are driven.
An aggravating problem is that, when foods or other products which must remain clean are packaged, the drives must be isolated from the hygienic area to which the food is exposed during packaging. If separate drives for the horns and anvils are located on both sides of the sealing area, both drives must be isolated from the sealing area. The same barriers which maintain hygienic conditions make it difficult to monitor the cartons being sealed. Interconnected drives for both the horns and the anvils are sometimes difficult to isolate from the hygienic area where food is sealed.
Yet another problem which must commonly be addressed when ultrasonic sealing equipment is designed is how to prevent the intentionally separate moving parts of the equipment from being welded together, should they come into contact accidentally or intentionally during a sealing operation. Another problem is how to prevent the auxiliary equipment associated with ultrasonic sealing apparatus, such as cooling apparatus, from vibrating itself apart or otherwise being damaged by stray ultrasonic vibrations transmitted within the machinery.
The anvils for ultrasonic sealing apparatus have also previously been provided with cooling apparatus for removing heat transmitted during the sealing operation.